1. Field of the Invention
The present invention relates to a cable for the transmission of electrical signals. In particular, the present invention relates to an electrical cable having one or more ribbon conductor assemblies for improved signal transmission properties.
2. Discussion of Background
Electrical cables and interconnects are important but frequently overlooked components of audio and video playback systems. Well-chosen cables and interconnects can help users get the best performance from their systems, whereas poor or incompatible cables result in poor performance. The terms xe2x80x9ccablexe2x80x9d and xe2x80x9celectrical cablexe2x80x9d are often used to refer to any wiring in an audio or video system, most commonly to a power cord that connects a component to a source of electrical power such as a wall outlet, or a conductor that carries a high-current signal from a power amplifier to a loudspeaker. The term xe2x80x9cinterconnectxe2x80x9d refers to a conductor that connects line-level signals in an audio or video system. For example, interconnects are used between source components (CD player, turntable, tape deck tuner) and the preamplifier, and between the preamplifier and the power amplifier of a typical audio system. For purposes of this specification, the terms xe2x80x9ccable,xe2x80x9d xe2x80x9celectrical cablexe2x80x9d and xe2x80x9cinterconnectxe2x80x9d are used interchangeably.
Ideal cables are xe2x80x9cneutral,xe2x80x9d that is, they transmit electrical signals essentially instantaneously and without imposing significant distortion or degradation on the signals. However, real-world cables have electrical resistance, capacitance, and self-inductance properties which color the signals transmitted through the cables. Therefore, discriminating consumers select cables for their compatibility with the other system components, and (in some cases) for their ability to enhance the overall sound or video output of the system. Similar considerations apply to cables used in high-end computer systems, servers, and other applications where distortion-free signal transmission is a priority.
Many different types of electrical cables are available. Cables are frequently shielded to minimize the effects of electromagnetic interference on the signal-carrying conductors. Typically, shields of metal foil or braided strands of electrically-conducting material enclose the signal-carrying conductors and are electrically connected to ground potential at one or both ends of the cable. As noted above, all cables exhibit some signal degradation due to the effects of resistance/impedance, capacitance, and inductance. The most important sources of signal degradation arise from the interaction between the individual strands of multi-stranded conductors and a phenomenon known as the xe2x80x9cskin effect.xe2x80x9d A well-known problem with multi-stranded cable is the tendency for the signal to jump from strand to strand if the cable is twisted. Each strand interface acts like a small electrical circuit that adds capacitance to the cable, resulting in degradation of the signal. In addition, the flow of electrical current sets up a magnetic field in each conductor which induces a signal in adjacent conductors and further degrades the signal.
The skin effect is a factor at audio and higher frequencies: because the self-inductance of an electrical conductor is greatest at the center of the conductor, higher-frequency signals encounter a lower-impedance path towards the outside of the conductor, which reduces the effective cross-sectional area of the conductor at those frequencies, which in turn increases the impedance of the conductor at those frequencies. Because higher-frequency signals encounter higher impedances than lower-frequency signals, the relative amplitudes of different-frequency signals are distorted during transmission. This problem is particularly evident for mixed-frequency signals: the greater the bandwidth of the signal, the greater the distortion.
Cable designers seek to minimize these effects and (particularly for cables used to transmit audio and video signals) optimize the effective bandwidth of their cables using a variety of techniques. For example, ribbon-type conductors are used in many applications. Brunt (U.S. Pat. No. 5,900,589) shows an audio transmission cable with a signal-carrying conductor of pure (or nearly pure) silver ribbon enclosed by an insulating material, where the width of the conductor is at least five times its thickness. A ground conductor lies alongside the ribbon conductor, placed so that the cross-sectional width of each the two conductors lies facing the other. The resulting assembly is enclosed by a second insulating layer, a conductive shield, and an outer insulating material.
Shah, et al. (U.S. Pat. No. 5,500,489) provide a thin, flexible cable for electronic retailing applications. Their cable includes three ribbon conductors on one side of a dielectric ribbon, and a flame-resistant, electrically-insulating jacket enclosing the ribbon with the conductors.
Haldeman, Jr. (U.S. Pat. No. 3,586,757) discloses a flexible stripline transmission line having individual conducting and insulating portions which are free to move relative to each other. The stripline consists of a pair of ribbon conductors sandwiched between three insulators, with a flexible outer cover that holds them in place. There appears to be a small air space on each side of the conductor/insulator stack.
Eisler (U.S. Pat. No. 3,317,657) shows several designs for flat electric cables for heating applications. The cables include flat conductor strands, each enveloped in a sheath of insulating film. A plurality of such sheaths are secured to a wider insulating film by an adhesive or by welding.
Hoover""s flat cable (U.S. Pat. No. 2,200,776) has three pairs of insulated copper ribbons that serve as power conductors, and two additional insulated conductors for connection to motor control devices. The ribbon conductors are enclosed by fiber insulation and linen tape; the cable structure is enclosed by a copper tube.
Weaver (U.S. Pat. No. 2,060,913) and Guilleaume (U.S. Pat. No. 531,614) provide telephone cables. Weavers self-coiling cable includes one or more strands of an elastic material such as phosphor bronze alloy which can be wound in a resilient helix. The conductors are insulated and bound together by braided textile covers or otherwise. Guilleaume""s telephone cable has several strands of ribbon conductors twisted together and sheathed with paper or other insulator, followed by an outer sheath of lead. The ribbon conductors in each strand are insulated from each other by paper.
Trazyik (U.S. Pat. No. 5,872,334) and King (U.S. Pat. No. 2,586,345) use conductive shields in their devices. The Trazyik high-speed cable includes a pair of cables, each having a copper wire core surrounded by a dielectric layer (such as PTFE), a ground conductor, and a conductive shield. The cables are encased in a polymeric jacket, which is impregnated with a conductive material such as carbon. King discloses a three-core paper insulated mine shaft cable. The cable has three sets of conductors, each set consisting of a plurality of metal strands surrounded by paper insulation impregnated with micro-crystalline petroleum wax and cable-impregnating mineral oil. The conductor sets are encased in (in sequence) a metallized paper screen, a lead sheath, a layer of jute, steel wire armouring, and another layer of jute.
Despite the many types of cables available to consumers, there is a need for a wide-bandwidth, low-distortion electrical cable that can be used as a power cable or interconnect cable in a variety of different applications (audio and video systems, home theater systems, computer systems, servers, etc.). Such a cable would have a low overall impedance combined with low self-inductance, and, optionally, a dedicated ground conductor.
According to its major aspects and broadly stated, the present invention includes a ribbon conductor assembly having a ribbon conductor with an approximately rectangular cross-section transverse to a longitudinal axis of the cable, a dielectric jacket enclosing the ribbon conductor, and an electrically-conductive shield. A single-conductor electrical cable (also termed herein a xe2x80x9cribbon cablexe2x80x9d) includes one such ribbon conductor assembly; a multi-conductor electrical cable includes two or more such ribbon conductor assemblies positioned so that the dielectric jacket of each ribbon conductor assembly contacts the dielectric jacket of at least one other ribbon conductor assembly, with all the ribbon conductor assemblies preferably enclosed by a single electrically-insulating casing. The ends of the electrical cable may be connected to any suitable male or female connectors or terminators, including single-ended interconnects such as RCA plugs, balanced interconnects such as 3-pin XLR connectors, IEC connectors, banana plugs, two-prong and three-prong plugs or jacks, and so forth.
The ribbon conductor is an important feature of the present invention which results in surprising performance advantages over standard solid core or stranded conductor configurations (including those with circular (or approximately so) cross-sections). The ribbon conductor is made of a low-resistance material, preferably copper, silver, aluminum, conductive carbon, electrically-conductive composite materials, or mixtures or alloys thereof. While these materials are generally preferred, other electrically-conductive materials may also be useful for some applications. As used herein, the term xe2x80x9cribbon conductorxe2x80x9d refers to a conductor whose transverse cross-section has a greater width than its thickness, that is, the cross-sectional width of the conductor is greater than its cross-sectional thickness. This ribbon configuration results in a lower self-inductance which enhances current flow through the conductor, resulting in lower signal distortion and concomitantly enhanced sound and video quality. The ends of connectors attached to the ribbon conductor may be plated with a selected metal or alloy (different from the metal of the connectors themselves) to further optimize current flow and/or adjust the quality of electrical signals transmitted through the conductor.
The dielectric jacket is another feature of the present invention. The jacket clamps the ribbon conductor to hold it firmly in place with air gaps on either side. Clamping the edges of the ribbon conductor provides effective damping effects without over-damping: damping controls resonance by reducing mechanical vibrations and thereby improves the frequency response of the cable. This effect is especially evident at lower frequencies, thus, the electrical cable has an improved bass response when used to transmit audio-frequency signals.
For good signal transmission through a conductor, it has now been determined that the best dielectric is no dielectric at all. That is, the optimal environment for current flow through a conductor is present when the conductor is surrounded by air (or vacuum). The dielectric jacket of the present invention secures the ribbon conductor and also forms air gaps on both sides of the conductor, ensuring that the conductor is substantially surrounded by air for the best possible signal transmission properties.
The shield is still another feature of the present invention. The shield is adjacent to the dielectric jacket of the ribbon conductor assembly, and may be applied by spraying the inner or outer surface of the jacket with metallic paint containing particles of copper, silver, gold, aluminum, carbon, graphite, or other electrically-conducting materials, or alloys or mixtures thereof, in a suitable carrier. Alternatively, the shield may be applied by coextruding a suitable electrically-conducting material with the jacket material, depositing a layer of electrically-conductive material on the jacket, or installing a foil or braided wrapping adjacent to the jacket. In a multi-conductor electrical cable constructed according to the invention, electrical contact between the shields of the individual ribbon cable assemblies optimizes the overall impedance and improves the signal-transmission properties of the cable.
The interaction of the shield with the ribbon conductor to improve signal transmission is another feature of the present invention. Current flowing in the shield produces an electron density containment field (xe2x80x9cEDCFxe2x80x9d) that is believed to xe2x80x9csqueezexe2x80x9d and contain the field produced by current flowing in the ribbon conductor. This effect is especially evident when the shield is adjacent the inner surface of the jacket, where the EDCF appears to densify the field produced by current flow in the ribbon conductor, thereby improving audio signal transmission.
The versatility of the electrical cable is yet another feature of the present invention. An electrical cable according to the invention can have one, two, or more ribbon conductor assemblies as described above, each ribbon conductor assembly with dimensions that depend on the particular application. For example, a three-conductor electrical cable may have three ribbon cable assemblies, one of which may serve as a ground conductor. The cable ends can be fitted with a variety of connectors or terminators, whether known in the art or to be developed, and the materials for the various components of the cable can be selected to optimize its electrical signal transmission properties for a particular application.